In order to produce aluminum, aluminum oxide (alumina) is molten at high temperature in a bath comprising cryolite and other fluorine salts. Molten aluminum oxide decomposes in an electrolytic cell under the action of direct current passing through the bath. The pure aluminum deposits on the cathodes, while the oxygen oxidizes and consumes the anodes.
With time, the amount of alumina in the bath decreases. When the alumina concentration in the bath decreases to 0.5-1.5%, a special mode of cell operation occurs known as the anode effect, which is accompanied by low level of the cell performance.
During operation, to maintain satisfactory cell performance new doses of alumina are added to the melt at regular time intervals or alternatively continuously supplying alumina by alumina feeding mechanisms (AF mechanisms).
It is known that in order to efficiently control the electrolytic process it is necessary to stabilize the power conditions and material balance thereof. Among the objects of the method of controlling electrolysis of aluminum is achieving the best cell performance, and for this purpose, establishing optimum conditions for dissolution of alumina and stabilizing the alumina concentration under variable process parameters, quality of raw materials, etc. Among known methods of process control for aluminum cell operation are the methods which are based on the normalized voltage and the direction of anode movement characteristics
A method of automatic control of an aluminum cell comprises the steps of measuring cell voltage and cell current, calculating current values of the cell resistance, establishing the baseline value of resistance or normalized voltage before addition of each dose of alumina. The minimum value of normalized voltage Unormmin is used as a process control set point. In case of discrepancy between the normalized voltage Unorm and current value of normalized voltage Unorm rcur., the anode is moved and all such movements are recorded. When the anode movements are mostly in the downward direction, the concentration value of the cell is set as decreasing. When anode movements are mostly in the upwards direction, the concentration value is set as increasing. When anode movements are rare, immediately after works and before the next handling of the cell, the concentration value is set as normal. To achieve normal conditions for alumina concentration in the bath when the concentration values are increased and decreased, the alumina loading mode is corrected by addition of alumina or by other actions (See RU, 2148108).
Among major drawbacks of these prior art methods is that they do not allow timely feedback of the cell disturbances which result from changes in the electrolytic characteristics and the overall quality of raw materials. This ultimately reduces operational efficiency.
Among known methods of controlling an aluminum cell are the normalized voltage method and the mathematical modeling method. These methods apply the techniques of controlling an aluminum cell by periodic feeding of alumina into the cell, measuring the cell voltage and line current, calculating bath resistance across the anode-cathode space, determining the average value of these parameters and determining concentration of the alumina in the bath by a mathematical model, and calculating the variation of alumina feeding rate into a cell by deviation of the calculated concentration from the set value. (See Russian Patent Documents RU 2106435 and RU 2204629).
The major disadvantage of these methods is the inherent inaccuracy of the alumina concentration due to inadequacy of responsiveness of the used model to many important operational characteristics, including changes in bath temperature, quality of alumina, etc. This results in low level of stability and high fluctuations of alumina concentration in the bath.
The known methods of controlling aluminum cells further include equipping aluminum cells with point-feeding system controlled by normalized voltage and derivatives thereof. For example, a control method is utilized which maintains the temperature conditions of a cell by alternating overfeed and underfeed modes and adjusting the anode-cathode distance and alumina concentration within preset values. The method comprises measurement of cell voltage and line current, calculation of current value of normalized voltage Unorm and the rate of changes thereof in time dUnorm/dt, comparison of calculated values with preset values and making a decision to adjust the anode-cathode distance and shifting from overfeed to underfeed modes or vice versa on the basis of such comparison (See Russian Patent Documents SU 1724713 and RU 2113552).
These methods are capable of maintaining the alumina concentration in the bath in the technologically acceptable range between 2 and 5%. However, in application of these methods the feeding mode criteria, such as a preset time of overfeed mode and maximum voltage of underfeed mode do not provide the required accuracy for maintaining the alumina concentration. In these methods only the rough estimates of dUnorm/dt (positive/negative) are utilized.
It appears that the most relevant prior art method to the present invention is that of controlling aluminum production cells by maintain the temperature levels through alternating the overfeed and underfeed modes and adjusting the anode-cathode distance and adjusting the alumina concentration within preset values. There are the following steps in this prior art method: measuring the cell voltage and line current, calculating the current value of normalized voltage Unorm and the rate of its changes over time dUnorm/dt, comparing the calculated values with the preset values and making a decision concerning the adjustment of the anode-cathode distance, so as to shift the method either to overfeed or underfeed modes. This occurs on the basis of comparison of the variation rate of normalized voltage value dUnorm/dt>G1 over time. Transition from the overfeed mode to underfeed mode occurs when the normalized voltage variation rate over time is dUnorm/dt<G2, where G1 and G2 are the threshold values of normalized voltage variation rate which are established experimentally. Furthermore, the anode-cathode distance is adjusted upon transition from the overfeed mode to underfeed mode, provided: |Unorm−Uo|>ΔU, where U0 is the nominal value of normalized voltage, ΔU is predetermined by the process requirements a zone of insensitivity (See Russian Patent Document RU 2189403).
In the prior art method, the base mode is the mode which takes place at the moment of transition from the overfeed mode to the underfeed mode. The base mode time is calculated based on the cell output (daily alumina doses). In this mode the pseudo resistance of the cell is almost constant. Therefore, at this specific time, the anode beam position is corrected.
Among major drawbacks of this prior art method is that it does not take into account changes in the alumina concentration in the bath resulting from changes in the quality of raw materials, electrolytic parameters, and characteristics of the alumina feeding device, i.e. alumina dose mass. Furthermore, the rate of dUnorm/dt responds only to the concentration of alumina dissolved within the bath. When the rate of alumina dissolution decreases during the high level massive alumina feed (i.e. the overfeed mode), the dUnorm/dt rate is of considerable importance, as it indicates about the necessity to add alumina into the pot. This occurs even though such addition is not required. Thus, the alumina point-feeding system control often responds in the way which is opposite to the required one. Thus, in highly undesirable fashion, this prior art system carries out massive loading of alumina during the overfeed mode. In response the pot becomes overburdened with alumina, and alumina muck is formed at the bottom of the cell. As a result, the actual fluctuations of alumina concentration in the bath become impermissibly high. This causes increased frequency of the anode effects and increased frequency of turning on the motors adapted to move the anode carbon (i.e. squeezing of the pot). This increases the number and severity of process malfunctions and causes deterioration of the cell performance. The latter results in the increase of power consumption, the decrease of cell output, and the increase in labor to eliminate the process problems.
Inability to obtaining the desired technological results arises because the algorithm of the prior art method does not take into accounts variables such as the changes in the quality of raw materials, the rate of alumina dissolution, and the characteristics of electrolysis and the point-feeding system.